Cesium, or caesiuma, is the chemical element with atomic number 55, symbol Cs.

Under standard conditions, the simple cesium body is a soft and ductile metal, white or silver to golden. Its melting point (28 กใ C) is close to room temperature and to the human body (CNTP), at which it can remain in the liquid state by supercooling; cesium shares this property with gallium and rubidium, mercury being the only pure liquid metal and evaporating at room temperature.

Cesium is the most alkaline of the alkali metals (strongest known base; enough to attack glass). Its chemical properties are close to those of rubidium and potassium, which belong to the same family. It reacts violently with water and air (explosive reaction). It is extremely reactive and pyrophoric, and reacts with water even at a temperature as low as -116 กใ C.

It is extracted from the pollucite (aluminosilicate Cs4Al4Si9O26 กค H2O) but it is also present in trace amounts in lepidolite

Cesium is the least electronegative (most electropositive) of metals (apart from francium, but the latter is radioactive with a short half-life), which makes it interesting for the photoelectric effect9.

It has two degrees of oxidation: 0 (metal) and + I (ionic compounds) 9.

Its three main types of compounds are9:

halides (fluoride, chloride, bromide, iodide);
carbonate and hydroxide;
salts of organic acids (including formate).
Reactivity: cesium reacts violently with air (production of superoxide), with water even frozen (explosion, which is inhibited only from −116 กใ C), with phosphorus, sulfur and dichlor

409 isotopes of cesium are known, with a mass number varying between 112 and 151, plus 17 isomers. This number of isotopes is one of the records for a chemical element9.

The only stable isotope is 133Cs (78 neutrons), which makes cesium a monoisotopic element. This isotope being the only one present in nature, cesium is also a mononucleidic element.

The other isotopes are radioactive and radiotoxic9. They are produced in supernovae, and on Earth are the result of fallout from atmospheric nuclear tests or nuclear accidents (Chernobyl disaster, for example) and nuclear waste, notably cesium 137. Two isotopes, 134Cs (half-life two years) and 137Cs (gamma emitter, half-life of 30 years) are fission products of uranium9, possible tracers of leaks or accidents from nuclear reactors.

The most well-known radiogenic isotope 137Cs. It has indeed been widely used in the hydrological and ecological studies implemented to evaluate the effects of several general contamination of the atmosphere induced from 1945 by the use of atomic bombs, then by nuclear tests, then (in the northern hemisphere mainly) by the Chernobyl accident) and finally to a lesser extent, due to releases from nuclear power plants or reprocessing sites, storage, etc. Its signature and monitoring have, for example, made it possible to measure how quickly groundwater is being renewed or to demonstrate that a cave was or was not isolated from the outside world. It has also been used to study the environmental kinetics of cesium (especially in the food chain). It has a radioactive half-life of 30.15 years. It disintegrates into 137m barium, a short period isomer (2.55 min) which disintegrates itself into stable 137 barium. Its standard atomic mass is 132.9054519 (2) u.

Radioactive waste, fallout from atmospheric nuclear tests or the Chernobyl accident may contain cesium 135 with a very long radioactive period, cesium 134 (2 year period), and cesium 137 (30 year period). Cesium 135 has a radioactive period of 2.3 million years and is part of long-lived radioactive waste


rhenium    germanium    zirconium     cadmium     hafnium

      barium   lithium     beryllium     strontium     calcium

      Tantalum    gadolinium    samarium      yttrium   ytterbium

       Lutetium    praseodymium   holmium     erbium   thulium     dysprosium

       terbium   europium  lanthanum   cerium   neodymium  scandium 

         rubidium    cesium